Regents Biology
Ch. 19 - Viruses
Overview: A Borrowed Life Viruses called bacteriophages can infect and
set in motion a genetic takeover of bacteria, such as Escherichia coli
Viruses lead “a kind of borrowed life” between life-forms and chemicals
The origins of molecular biology lie in early studies of viruses that infect bacteria
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Regents Biology
Figure 19.1
0.5 mm
Regents Biology
Concept 19.1: A virus consists of a nucleic acid surrounded by a protein coat
Structure of Viruses Viruses are not cells A virus is a very small infectious particle
consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope
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Regents Biology
Viral Genomes
Viral genomes may consist of either Double- or single-stranded DNA, or Double- or single-stranded RNA
Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus
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Regents Biology
Capsids and Envelopes
A capsid is the protein shell that encloses the viral genome
Capsids are built from protein subunits called capsomeres
A capsid can have various structures
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Regents Biology
Figure 19.3
Capsomereof capsid
RNA CapsomereDNA
Glycoprotein Glycoproteins
Membranousenvelope RNA
CapsidHead
DNA
Tailsheath
Tailfiber
18 250 nm 80 225 nm70–90 nm (diameter) 80–200 nm (diameter)
20 nm 50 nm 50 nm 50 nm(a) Tobacco
mosaic virus(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4
Regents Biology
Some viruses have membranous envelopes that help them infect hosts Viral envelopes surround the capsids of
influenza viruses and many other viruses found in animals
Derived from the host cell’s membrane Contain a combination of viral and host cell
molecules
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Bacteriophages, also called phages, are viruses that infect bacteria Have the most complex capsids
found among viruses Have an elongated capsid head
that encloses their DNA A protein tail piece attaches the
phage to the host and injects the phage DNA inside
http://scitechdaily.com/video-animation-on-how-a-flu-virus-works/
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Regents Biology
Overview: Masters of Adaptation Utah’s Great Salt Lake can reach a salt
concentration of 32% Its pink color comes from living prokaryotes
Ch. 27 - Prokaryotes
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Regents Biology
Thrive almost everywhere, including places that are too: Acidic Salty Cold/Hot
Most are microscopic, but what they lack in size they make up for in #s
There are more in a handful of fertile soil than the number of people who have ever lived
Prokaryotes are divided into two domains: Bacteria Archaea
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Prokaryotes
Regents Biology
Concept 27.1: Structural and functional adaptations contribute to prokaryotic success Most likely 1st organisms on Earth Most are unicellular, although some species
form colonies Sizes are usually 0.5–5 µm
Eykaryotic cells are usually 10–100 µm Come in a variety of shapes, the 3 most
common shapes are: spheres (cocci) rods (bacilli) spirals
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Regents Biology
Cell-Surface Structures
Cell wall: Maintains cell shape Protects the cell Prevents cell from bursting in a hypotonic
environment A eukaryotic cell wall is made of cellulose or
chitin Bacterial cell walls contain peptidoglycan
A network of sugar polymers cross-linked by polypeptides
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Regents Biology
Archaea contain polysaccharides and proteins but lack peptidoglycan
Scientists use the Gram stain to classify bacteria by cell wall composition Gram-positive bacteria have simpler walls with
a large amount of peptidoglycan Gram-negative bacteria have less
peptidoglycan and an outer membrane that can be toxic
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Regents Biology
Figure 27.3
(a) Gram-positive bacteria: peptidoglycan traps crystal violet.
Gram-positivebacteria
Peptido-glycanlayer
Cellwall
Plasmamembrane
10 m
Gram-negativebacteria
Outermembrane
Peptido-glycanlayer
Plasma membrane
Cellwall
Carbohydrate portionof lipopolysaccharide
(b) Gram-negative bacteria: crystal violet is easily rinsed away, revealing red dye.
Regents Biology
Many antibiotics target peptidoglycan and damage bacterial cell walls
Gram-negative bacteria are more likely to be antibiotic resistant
A polysaccharide or protein layer called a capsule covers many prokaryotes
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Regents Biology
Figure 27.4
Bacterialcell wall
Bacterialcapsule
Tonsilcell
200 nm
Regents Biology
Some prokaryotes have fimbriae, which allow them to stick to their substrate or other individuals in a colony
Pili (or sex pili) are longer than fimbriae and allow prokaryotes to exchange DNA
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Regents Biology
Figure 27.5
Fimbriae
1 m
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Motility
In a heterogeneous environment, many bacteria exhibit taxis: the ability to move toward or away from a
stimulus Chemotaxis is the movement toward or away
from a chemical stimulus Towards nutrients Away from toxins
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Regents Biology
Most motile bacteria propel themselves by flagella scattered about the surface or concentrated at one or both ends
Flagella of bacteria, archaea, and eukaryotes are composed of different proteins and likely evolved independently
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Regents Biology
Figure 27.6
Flagellum
Hook
Motor
Filament
RodPeptidoglycan
layerPlasma
membrane
Cell wall
20 nm
Regents Biology
Evolutionary Origins of Bacterial Flagella
Bacterial flagella are composed of a motor, hook, and filament
Many of the flagella’s proteins are modified versions of proteins that perform other tasks in bacteria
Flagella likely evolved as existing proteins were added to an ancestral secretory system
This is an example of exaptation, where existing structures take on new functions through descent with modification
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Regents Biology
Internal Organization and DNA
Prokaryotic cells usually lack complex compartmentalization
Some prokaryotes do have specialized membranes that perform metabolic functions
These are usually infoldings of the plasma membrane
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Regents Biology
The prokaryotic genome has less DNA than the eukaryotic genome
Most of the genome consists of a circular chromosome
The chromosome is not surrounded by a membrane; it is located in the nucleoid region
Some species of bacteria also have smaller rings of DNA called plasmids
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Regents Biology
Figure 27.8
Chromosome Plasmids
1 m
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There are some differences between prokaryotes and eukaryotes in DNA replication, transcription, and translation
These allow people to use some antibiotics to inhibit bacterial growth without harming themselves
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Regents Biology
Reproduction and Adaptation
Prokaryotes reproduce quickly by binary fission and can divide every 1–3 hours
Key features of prokaryotic reproduction: They are small They reproduce by binary fission They have short generation times
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Regents Biology
Many prokaryotes form metabolically inactive endospores, which can remain viable in harsh conditions for centuries
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Regents Biology
Figure 27.9
Coat
Endospore
0.3 m
Regents Biology
Their short generation time allows prokaryotes to evolve quickly
For example, adaptive evolution in a bacterial colony was documented in a lab over 8 years
Prokaryotes are not “primitive” but are highly evolved
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